In mechanical dishwashing generally two cleaning cycles, usually separated by intermediate rinsing cycles with pure water are used. In the two cleaning cycles, different products are utilized. In the first or true cleaning cycle, alkaline-reacting agents are employed for the loosening and emulsifying of the food residues. In the after-rinsing or clear-rinsing bath, on the other hand, special clear-rinsing agents are employed. The latter should possess a good wetting power and be able to reduce the surface tension of the after rinsing water to such a degree that it drains in a film-like manner from the dishes and leaves no visible deposits, such as lime spots or other impurities, and completely clear dry dishes are obtained. This is called "sheeting" and the clear-rinsing agents concentrates are often stated to have a "sheeting" component.
Because of the violent agitation of the liquor in the dishwasher, these clear-rinsing agent have to be as low-foaming as possible. The customary anionic wetting agents, however, such as higher-molecular weight alkyl sulfates or alkyl sulfonates or aralkyl sulfonates are not generally usuable because they foam too much. In practice, therefore, mostly nonionic tensides based on ethylene-oxide adducts to fatty alcohols, alkylphenols, or polypropylene glycols of higher molecular weights are employed. These products, however, were also found in actual practice to be not sufficiently low-foaming in the concentration range, required for a sufficient wetting effect.
These adducts have been found to cause disturbances due to excessive foam formation particularly in commercial dishwashing machines which have a very high rate of water circulation and a very high return rate of the clear-rinsing liquor into the main rinsing cycle. The same difficulties may also arise in home dishwashing machines. Even with the use of relatively low-foaming ethylene-oxide adducts, it is therefore necessary to add anti-foaming agents to the clear-rinsing agents. The substances used as foam suppressors or anti-foaming agents may be nonionic alkoxylation products which are relatively insoluble in water at the rinsing temperatures employed, that is, adducts of ethylene oxide onto higher alcohols or alkyl phenols having only a low degree of ethoxylation, or similar adducts of ethylene oxide and propylene oxide. However, at the temperatures employed, these products have no wetting action and are therefore a ballast to the clear-rinse agent. Moreover, they are in most cases not sufficiently biologically degradable.
The components of the clear-rinse agent should not only be characterized by good wetting and low foaming but the wetting agents used in them should also be biologically readily degradable and, as for as possible, non-toxic to the living organisms in the water.
Numerous clear-rinse agents which fulfil one or more of the four main requirements, namely efficient wetting, low foaming and/or biological degradability and/or low toxicity are known both in practice and in the literature but there is still a need for clear-rinse agents which will satisfactorily fulfill all four of these requirements. Furthermore, time has shown that a raw material once used is not always available in unlimited quantities so that the expert must constantly find alternatives that are at least equivalent.
In German Published Application DOS No. 2,110,994, clear-rinse agents for automatic dish washing have been disclosed which contain, as their sheeting component, adducts of propylene oxide and non-reducing sugars or sugar derivatives and in Great Britain Pat. No. 1,167,663, corresponding to DOS No. 1,628,642, clear-rinse agents have been disclosed which contain water-soluble starch degradation products and/or sugars as their discharge components. All of these agents are distinguished by sufficiently low foam formation and physiological acceptability and they produce a satisfactory clear drying effect.